The present invention relates to a vehicle control system having a transmission ratio between a steering wheel and front wheels that changes according to a running state of the vehicle.
An example of a conventional vehicle control system of this type is one in which a transmission ratio between the steering wheel and the front wheels changes according to the vehicle velocity (see Japanese Laid-Open Patent Publication Number Hei 11-91604, Section [0020]). In another example, the vehicle can switch between two-wheel drive and four-wheel drive based on the operating state (see Japanese Laid-Open Patent Publication 2003-127690, Section [0002] and FIG. 1).
In a vehicle control system developed by the present applicants, various information relating to the operating status of the vehicle is used to detect (or estimate) the grip factor of the front wheels and to determine if there is understeering. If there is understeering and an insufficient grip factor, the transmission ratio is changed so that the same steering operation magnitude results in a small amount of steering in the front wheels, thus preventing understeering from becoming more pronounced.
While the front-wheel grip factor can change in response to a change in the two-wheel/four-wheel drive status, the vehicle control system does not take changes in drive status into account. Thus, when there is a switch to two-wheel or four-wheel drive, a delay takes place in the transmission ratio, making it difficult to effectively limit understeering.